cyclegraph/model: unify, clean up graph printing

cyclegraph/model: unify, clean up graph printing

This fixes several things:

1) The rf edges now are directed properly
2) Read-from-promises show up in the graph
3) ModelChecker uses the same code as CycleGraph for printing a
   CycleNode or graph edge
4) Graphs are more readable; I put a large weight on the
   sequenced-before edges, causing graphviz to try to make these edges
   "shorter, straighter and more vertical" [1]
5) Use the 'SnapshotAlloc' STL allocator for CycleGraph::nodeList

[1] http://www.graphviz.org/doc/info/attrs.html#d:weight